Jet Engine

Piston engine is a constant volume cycle.

Jet engine is a constant Pressure cycle, whereas Piston engine is a constant volume cycle

Piston engine is a 5 event cycle & 4 stroke cycle.

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Jet engine is also a 5 event cycle.

                         1)   Induction

                         2)   Compression

                         3)   Ignition

                         4)   Power or Expansion

                         5)   Exhaust

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In a jet engine all events take place one after the other simultaneously.

In a piston engine one event finishes & then the next event starts.

In a piston engine combustion takes place intermittently, but in a jet engine combustion is a continuous process (Till engine is off).

Jet engine cycle is also known as continuous combustion cycle.

A propeller driven engine handles big mass of air & gives a small acceleration.

A jet engine handles a small mass of air & accelerates it to a larger velocity.

Piston Engine is an OTTO cycle.

Jet Engine is an BRAYTON cycle.

Jet engine is a constant pressure cycle or continuous combustion cycle.

Ram effect or Ram rise of Pressure

When air reduces in volume, pressure increases (velocity decreases, pressure increases)

Its effect is more prominent at speeds above 250kts.

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Jet Engine Parts.

1) Air Intake – draws air inside

2) Compressor – compresses air & decreases pressure (pressure increases therefore temperature increases also).

3) Combustion chamber – fuel mixed with air & burn.

4) Turbines – drives compressor

5) Exhaust nozzle – directs air out.

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1) Air Intake – It is an airframe feature not jet engine part. Air intake should be designed to give max Ram recovery without causing any loss of pressure, due to turbulence.

Therefore it should be smooth & straight without any bents.

In sub-sonic A/c air intake should be divergent in shape.

In Divergent passage

At Sub sonic air flow – Velocity decreases pressure increases

At Super sonic airflow – Velocity increases pressure decreases

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In Convergent passage

At Sub sonic air flow – Velocity increases pressure decreases

At Super sonic airflow – Velocity decreases pressure increases

 A subsonic flow when passes through divergent duct its velocity decreases, but when a super sonic flow passes through divergent duct its velocity increases.

In a supersonic A/c, initially A/c is operated at sub sonic speeds, while Take off and Landing phase of the flights. Therefore air intake design is made of Convergent/ Divergent shape and we have variable size intake.

At low speed ranges (below 0.8 mach no.) it is kept divergent, because velocity decreases at subsonic speed and pressure increases

At high speed ranges above 0.8 mach no.) it is convergent, because velocity decreases at supersonic speed and pressure increases.

Subsonic A/c uses Divergent duct, whereas a Supersonic A/c uses Convergent/ Divergent duct

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2) Compressor – is a main part of a Jet engine, it increases pressure of air.

There are 2 types

1)       Centrifugal           2)       Axial flow

1.)  Centrifugal compressor has

                         i – Impeller, which increases the velocity of air

                         ii – Diffuser, which decreases the velocity & increases the pressure

                         iii –  Manifold – stores the pressure

Centrifugal compressor has single entry or double entry air intake, one impeller & one diffuser.

Centrifugal flow compressor airflow is radial and for this reason we can have only up to 2 stages of compressor.

Single Entry the Intake for air, one Impeller & one diffuses.

Double Entry – Air enters from sides of the compressor.

It has one Impeller having strands on both sides. It has 2 diffusers but only one manifold.

When pressure is increased, there is an adiabatic rise in temperature.

Some energy is lost & cannot be used, because it is used in decreasing temperature.

The rise in temperature is independent of the pressure rise & it depends upon working capacity of the impeller.

Therefore practical increase in pressure is less than the theoretical increase.

Whereas in a Piston engine, the Practical rise in pressure is more than Theoretical rise, because of hot temperature of the cylinder walls.

Disadvantage

(1) Rise in pressure is low as pressure ratio is low (5:1).

To increase pressure we can put one compressor after the other, but this has a limitation since the air flow is not straight and it moves in a centrifugal shape, but even then pressure ratio does not increase very much.

(2) Frontal area is more therefore it produces lot of drag

Ram jet engine

Disadvantage

(1) It requires a vehicle to launch it to a particular velocity, but once launched it continues to run.

(2) As pressure of air increase it gives better output. Greater the pressure increase, greater the thrust output.

Pulse jet – Ignition is intermittent & at air intake we have flapper valves.

Flapper Valves – when ignition takes place they close preventing air from escaping from inlet.

After ignition they open again, allowing air to come in, ignition takes place in intervals therefore they are called pulse jets.

AXIAL FLOW COMPRESSOR

We have a combination of Stator (Stationary) & Rotors (Rotates), both are fitted on a common shaft.

Stator has 2 functions

1)  Reduces velocity to increase pressure

2)  It changes direction of airflow

(From Radial flow of air to linear flow, to make it flow along Axial length)

Angle by which Stator changes direction, it depends upon stalling angle of the next stage of rotor blades.

Stator provides air to the Rotor at on optimum angle of attack.

In the engine 1st stage is a STATOR & then ROTOR.

First stage of the STATOR is Inlet Guide Vanes.

The pressure rise is not only through the STATOR but Rotor also has some rise in pressure.

STATOR & ROTOR combination is called one stage of compressor.

In an axial flow compressor pressure rise per stage is kept small, because the pressure rise is a diffusing action which means decrease in velocity, but we don’t want large changes in velocity through the engine.

Therefore pressure rise per stage in Axial flow comp is less as compared to centrifugal flow compressor (5:1) to (20:1) axial flow, but overall rise in pressure is much more by having many stages, one after the another (20:1).

Therefore Axial flow compressor are very long.

A compressor’s stage (Difference between a Rotor & Stator), area decrease as we go back because as air is compressed it requires less space & moment you increase Cross Section area pressure decreases.

Through the compressor only the pressure changes, but not the velocity.

The velocity of air entering the compressor is same as leaving the compressor.

All the Rotors are being rotated by the “Turbines”.

Therefore all Rotors rotate at the same rpm because they are mounted on the same shaft, but rotors are handling different pressure.

Front ones have less pressure.

Rear ones have high pressure.

At low throttle settings front Rotors rotates at optimum rpm, but rpm of rear ones is not optimum, with reference to the high pressure being handled by them.

This occurs because as pressure increases, more energy is required.

1) To change direction of air.

2) Energy is required by rotors to push air back to the next stage.

Air starts piling up n the rear stages, it starts flowing in reverse direction (Ahead), reduces velocity of air through compressor and angle of attack of rotor increases & finally they stall.

Stalling 1st takes place in the High Pressure stages & then it advances to Low Pressure stages, when throttle is advanced, later all stages stall & rpm decreases this is called Compressor Surge/ Stall.

Causes for Compressor stall

1)   Suddenly open Throttle from low thrust to high thrust condition,

2)   Suddenly close throttle from high to low power settings.

3)   Whenever there is a major disruption of air flow through compressor.

Indications of Compressor stall

1)  Rise in EGT/ TIT/ ITT/ JPT/ TGT

EGT is taken after the combustion chamber before the 1st stage of Turbine.

EGT rises because air through combustion chamber decreases, but fuel flow remains same & therefore temperature increases (Unlike piston engine where if fuel flow is more and air is less temperature decreases.).

2)  Fluctuating Fuel flow

Fuel flow unit gets the input from

a)   Before combustion chamber

b)  Temperature from exhaust

3)    Fluctuating or decreasing RPM

4)    Loud Rumbling noise

5)    Loss in Thrust

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Results of Compressor stall

1)                 Loss in Thrust

2)                 High EGT

3)                 Land Rumbling noise from engine

4)                 No throttle response

5)                 Engine Vibration

6)                 Likely flameout of the engine

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Flame out is a condition in which engine fire extinguishes (Continuous combustion stops/or Ignition stops) during flight (Engine failure).

Recovery from Compressor stall

1)  Reduce throttle to Idle (Or Thrust lever to idle) & then increase it slowly.

2)  If the stall still persists. You may be required to shut down the engine & attempt a flight re-start.

Note: In some engine there is an altitude restriction up to which a flight restart can be attempted because of low density of air at high levels. In that case descent to specified level.

Ram pressure depends upon speed, speed increases Ram pressure increases.

Ram engine has no compressor. It uses compressed air by Ram effect.

In a Supersonic A/c above the speed of sound entire air is bypassed from the compressor.

Therefore, efficiency of engine increases, because no energy of turbine is used to compress air, only Ram rise of pressure is used.

In supersonic flow when it passes through convergent duct velocity decreases pressure increases.

Piston engine is a constant volume cycle – Ignition takes place of a constant volume.

Jet engine is a constant pressure cycle – this pressure is different for different throttle setting but for a particular throttle setting pressure is constant at which ignition takes place.

Whatever is the increase in velocity of air in Rotor same is decreased by STATOR therefore velocity of air entering ROTOR is same as velocity of air leaving the STATOR.

Innovations:

1)  By providing A/c bleed valve between 2 stages.

Therefore, when pressure is decreased in High Pressure stages, bleed valves open & release excess increase of pressure.

In low thrust conditions they are always kept open.

2) Twin Spool compressor

Compressor is divided into two parts

Low Pressure Compressor

High Pressure Compressor

Both compressors are run by different turbines

Front turbine runs at higher rpm.

Rear turbine runs at lower rpm.

Low pressure compressor is run by front turbine.

High pressure compressor is run by Rear turbine.

Low pressure compressor — N1

High pressure compressor — N2

It takes less energy to compress 1lbs of cold in to 1lbs of warm air to the same pressure.

As altitude increases, temperature decreases therefore less energy is required by compressor to compress air to the same pressure, but energy from turbine is same therefore compressor runs at increasing rpm in cold air & we don’t want this to happen. Therefore N2 rpm is governed by fuel flow therefore as altitude increases decreases fuel flow to maintain rpm (by not allowing it to increase).

By controlling N2 rpm we try to control N1 rpm, but as altitude increases, because of decrease in OAT N1 rpm increases.

As altitude increases density decreases, mass of air flow through engine decreases therefore thrust will decrease this is offset by increase in rpm.

This increase in N1 rpm cause a greater mass of air to move through compressor.

3)                 Variable angle Intel guide vanes – by controlling angle of attack of Rotor blades.

4)                 Variable angle Rotor blades

5)                 Variable angle Stator blades.

6)                 Acceleration control unit in FCU.

By decreasing fuel flow, N2 rpm decreases (maintained), because less air goes rough combustion chamber, therefore  less hot air, therefore High Pressure turbine speed decreases, this in turn decreases speed of Low Pressure turbine and therefore RPM of N1 RPM also decrease (but OAT still increase N1 rpm).

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Question :- Why a propeller gets affected by compressibility error at low rpm, whereas compressor of a jet engine rotates at much higher rpm and is still not affected by the compressibility affect.

Answer :- Propeller operates in an atmosphere where temperature is very low, therefore speed of sound decreases and Mach no of propeller decreases. Whereas in a Jet Engine Compressor due to increase in pressure, temperature increases due to adiabatic rise in temperature, with in the compressor the temperature is very high therefore Local Speed of Sound increases, and Mach No increases.

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Compressor increases pressure, this pressure is used to produce Thrust, because for same volume of the air, the weight of air is more due to compression.

Diffuser increases pressure by decreasing velocity utilizing venturi principle.

2 compressors are provided to prevent Compressor Stall or we want to run 2 compressors at different speeds to prevent Compressor Stall.

Rotor blades stall during compressor stall.

%age of max RPM — calibration of RPM gauge. (Tachometers)

High Pressure gauges have more %age, than Low Pressure gauge.

High pressure Turbines rotates at higher speed than low pressure Turbines.

70% of N2 rpm & 80% of N1 rpm does not mean N1 rpm is more than N2 (because max value for N1 & N2 is different).

All calibrations are done with reference to N2 rpm. If N2 rpm decreases on its own, it is an emergency.

Indications of Compressor Stall-

  1. RPM decrease.
  2. EGT rises or fluctuates rapidly.

Indications of Engine Failure-

  1. RPM decreases
  2. EGT decreases.

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3). COMBUSTION CHAMBER

Air is mixed with fuel & burnt.

Therefore Due to increase in temperature kinetic energy of gases increase with in the combustion chamber.

Types

1)   Annular

2)   Can Annular

1)   Annular – Cylindrical in shape with many fuel nozzle but 2 or 3 ignitors.

Advantage – They are smaller in length.

Disadvantage – For service, the whole engine has to be removed from A/c.

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2)  Can – It has many small cylinders (Cans) each can has one fuel nozzle, but only 2 or 3 cans will have ignitions.

Advantage of Can type – Individual cans can be removed for servicing, Engine is not required to be removed.

Disadvantage – They are longer in length because the diameter of each can is less and the cross section area is less to increase the cross section area, they are kept longer, therefore to handle more air, length has to be more.

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As air enters the cans it has to travel a certain distance with in the Can, before it is ignited therefore some pressure loss takes place.

In case of malfunction of a can, a severe temperature differential may cause vane distortion.

Behind combustion chambers, there are turbine blades rotating when they come in front of the malfunctioning. Cans they encounter cold temperature, but when it comes in front of perfectly working can it encounter hot temperature. Therefore each blade experiences a temperature differential during each rotation. Therefore they may get distorted.

With in the combustion chambers zone the Swirl vanes are provided to prevent flame from extinguishing, because they are operating at high velocity of air within the combustion zone.

Swirl vanes cause swirling action to prevent extinguishing of flame.

Modern A/c has a combination of CAN -Annular type.

Ignition in a jet engine is a continuous process, it come on automatically during the starting cycle & remains one throughout the flight.

Flame out – Engine flames out during flight.

It is caused due to disruption of airflow through combustion chambers.

Causes-

1)                 Compressor stall

2)                 Operating engine in low thrust conditions for long time

In low thrust conditions the length of flame decreases and if you suddenly open power, air flow stops the flame.

In jet engine thrust levers are never brought to zero or idle position during flight (Min. may be 20%).

In the combustion chamber 1st fuel is mixed with air & then ignited.

In critical phases of flight Take off, landing, passing through turbulence etc. the Ignitor switches are kept at ON position.

During flight restart, Igniters are turned ON.

With in the combustion chamber, the temperature is max in combustion zone.

Heat generated in the combustion chamber is not allowed to dissipate to the other parts of A/c.

Cooling of combustion chamber is by circulating ratio of air outside combustion chamber via bypass chambers.

Air is a bad conductor of heat & therefore provides on Insulator action not allowing heat to escape.

This is achieved in Bypass engine – Entire output of air of compressor is not passed through combustion chamber but a part of it is made to go over combustion chamber as well.

Primary Air – Going Inside the combustion chamber

Secondary Air – Going over the combustion chamber

By pass air – is Secondary Air.

This secondary air, cools walls of combustion chamber & cools.

By pass engine is more efficient then a turbo-jet

1)  Air going through combustion chamber is less, to get same fuel/air ratio, amount of fuel can be decreased, causing fuel flow to decrease.

2)  Engine makes less noise because velocity of exhaust gases decreases, because part of air going over combustion chamber has less velocity only the velocity of primary air (In combustion chamber) which is a very less part has high velocity. Both primary & secondary air meets at the end of combustion chamber before entering turbines. Therefore, Resultant velocity is less causing less noise.

3)   Turbines run at high temperature, therefore they require material of high tensile strength, which depends upon temperature at which turbines are required to run.

In a by pass engine because relative cold air is mixed with relative hot air (primary) therefore temperature of air decreases, turbine material can be of lesser tensile strength.

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4). TURBINES

Behind the combustion chambers the Turbines are located.

Turbines are designed to extract enough energy from escaping gases to run the compressor, leaving the balance energy to produce residual thrust.

Turbines also have a STATOR and a ROTOR combination.

Purpose of STATOR is to guide the airflow to the Rotor at an optimum angle of attack.

1st stage of STATOR after combustion chamber is called N.G.V.

N.G.V. = Nozzle guide vanes.

Types

1)   Impulse

2)   Reaction

1)    Impulse – area of turbine is uniform & air doesn’t increase in velocity.

Turbine rotates due to the impulse action (Due to Windmill).

Area of Nozzle Guide Vanes of Impulse turbine creates a venturi, therefore when air passes through it, it increases in velocity

Therefore, In Impulse turbine air accelerates through STATOR (Nozzle guide Vanes) but not through Rotors & vice versa in Reaction Turbines

Impulse Turbines run at a lower temperature, because as air passes through STATOR velocity increases, pressure decreases & temperature decreases.

2)   Reaction Turbines are of an aerofoil section, therefore air accelerates, when going through them. It produces total reaction & therefore lift (Radial lift which rotates the blade).

Nozzle Guide Vanes of Reaction turbines have a uniform area of Cross Section.

Reaction Turbines run at higher temperature, because as air passes through Stator velocity decreases, pressure increases and temperature increases.

Modern A/c has a combination of Impulse and Reaction called Impulse Reaction turbines.

Turbines – When air passes through Turbines – Temperature decreases, pressure decreases & therefore velocity decreases, because you are extracting energy.

In turbines gases expand, therefore blades are of Divergent sections.

Turbines undergo very high Thermal & Centrifugal stresses.

Stresses on Turbine

1)   Thermal

2)   Centrifugal

3)   Bending

4)   Compression

5)   Torsion

Turbine Creep – is the lengthening of turbine blades under the influence of high Thermal & Centrifugal stresses.

After Landing jet engine is not switched off immediately, but it is run for some time on Idling power Cos ®

Because the area of casing is more, it cools down faster but blades (having less surface area) cools down after some time & this can cause their rubbing against the case.

To prevent this to happen we don’t switch off engine immediately and run the engine on idle power for some time before switching off.

Thrust levers controls – Fuel flow to the Combustion chamber.

Throttle levers controls – Air flow to the Carburettor.

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5). Exhaust The purpose of Exhaust cone is to give a maximum increase in pressure to give a Ram effect, while decreasing velocity from the gases received from the Turbines.

The max air velocity an Axial flow compressor corresponds to is Mach. 0.9.  Whereas the velocity an Centrifugal compressor corresponds to is Mach. 1.2.

Centrifugal

  1. Impeller
  2. Diffuser
  3. Manifold

First stage of Compressor stator is also known as – Inlet Guide Vanes.

First stage of Turbines Stator is also known as – Nozzle Guide Vanes.

Compressor Rotor – increases velocity decreases pressure

Stator decreases velocity increases pressure

Turbine – Impulse turbine – has Stator, which increases velocity and decreases pressure

Rotor increases velocity decreases pressure

Reaction turbine has Stator, which decreases velocity and increases pressure

In static condition 2.6 lbs. of thrust is equivalent to be 1 H.P.

Calorific value of Petrol- 76,000 Centigrade Heat Unit./gal.

Calorific value of Kerosene – 83,000 Centigrade Heat Unit./gal.

Turbine and propelling nozzle give rearward thrust (Venturi effect)

Compressor, combustion chamber, exhaust unit and jet pipe gives forward Thrust.

When Engine is Static – gross thrust = Net thrust.

When Engine / A/c is in motion – Net thrust is less then Gross thrust.

Choked Nozzle– Choked Nozzle is when airflow at the nozzle reaches speed of sound.

Water methanol is used in turbo-propeller a/c during Take off for restoring S.H.P.

This power is known as Wet Power.

Wet Power is with use of Water Methanol.

Dry Power is without using Water Methanol.

On full power Water methanol is never switched OFF, because it will cause compressor to surge.

Concorde is the only commercial transport using Reheat.

Reverse Thrust – changes air direction by 135o.

Max thrust available in reverse thrust is approximately 50% of forward thrust.

A successful start of jet engine is indicated by a rise in EGT.

For starting a Jet Engine-

STARTER

IGNITION

FUEL

On putting H.P. cock forward, Ignition takes place & then fuel goes in.

HOT START- Fuel is more, air is less during starting, and temperature increases due to extra fuel causing engine damage. It causes Exhaust Gas Temperature to rise. It takes place during hot weather when outside temperature is high, because air density is low and fuel flow is more.

 WET START– Fuel fails to ignite and comes out from the jet pipe during starting. It does not cause Exhaust Gas Temperature to rise

FALSE START– Cranking the engine without fuel & ignition, usually done to clear off any fuel left in the Combustion chamber after WET START

HUNG START– During starting if rpm does not increase beyond Idling rpm and remains there only

On starting H.P. compressor is started first, because the weight of H.P is less.

In jet engine A/c. As speed increases, thrust is constant but T.H.P. increases.

In a propeller A/c power remains same but thrust decreases with increase in speed. (Therefore, propeller efficiency decreases).

Exhaust gives only 10-15% of total thrust and even lesser in case of a propeller driven jet engine.

Max thrust is given by Compressor.

All parts of jet engine produce Thrust

Therefore Newton’s 3rd law is applicable to all parts of a jet engine.

Newton’s 2nd law creates a Momentum.

GROSS Thrust is the momentum of outgoing gases.

Net Thrust is the Momentum of outgoing gases – Momentum of Incoming gases – Momentum of fuel.

Some decrease in pressure takes place at all parts of jet engine, which causes a decrease in thrust but this is overcome by the Momentum of fuel.

Therefore Net thrust = Momentum of outgoing gases – momentum of Income gases only.

Gross Thrust is equal to Net thrust, when A/c is stationary on the ground, because the Momentum of Incoming gases are zero.

Thrust Specific Fuel Consumption is the weight of fuel consumed per hour ÷ lbs of Thrust.

It should be as lower as possible

Factors affecting T.S.F.C.

1) Airspeed – As airspeed increases, Net thrust continues to decrease but gross thrust continues to increase due to RAM effect.

Therefore Total Thrust increases

Thrust decreases till 250 kts. & then it increases.

2)  Mass Airflow – Mass of Airflow going through engine changes the produced Thrust, Greater the mass airflow, greater the momentum & great the Thrust.

RPM increases mass airflow increases, jet engines are operated at high rpm.

RPM 100% on Take off

95-97% for Climb

90-95% for Cruise

RPM of operations depends upon Outside Air Temperature.

RPM of jet engine is given in %age of max rpm.

3)  Temperature- Temperature increases, mass air flow decreases, power output decreases.

4)  Density- Density increases, mass airflow decreases, power output decreases.

5) Altitude- Altitude increases thrust output decreases, because density decreases & mass airflow decreases & therefore total thrust decreases but Compressor Efficiency increases.

Because-

1)  Compressor efficiency increases in cold temperature.

2) Progressively lesser & lesser fuel flow is required, therefore T.S.F.C. decreases.

Best L/D speed gives best airframe performance.

At Vmd thrust requirements are minimum, but this speed is very less & controllability problem arises.

Therefore Jet engine is flown at IAS of 1.30% of Vmd.

If we maintain speed of 1.30% of Vmd, thrust required is less.

So therefore RPM decreases efficiency decreases, therefore we continue to increase to an altitude at which 90-95% rpm will give IAS that is 1.30% of Vmd.

In a jet engine, airframe is best when TAS/DRAG ratio is high.

With increase in altitude flying at IAS of 1.30% more than Vmd drag remains same, but TAS increases therefore TAS/DRAG ratio increases therefore Airframe efficiency increases.

Therefore, Jet engine are efficient at high altitudes, because engine & airframe efficiency increases with increase in altitude.

Jet Engine A/c’s are flown just below Tropopause.

Mach Critical- As we climb, TAS increases, speed of sound decreases & M. no. increases.

Critical Mach no is the Speed at which airflow over wings or any part of a/c reaches local speed of sound or Mach ONE.

Above Critical Mach no when velocity increases from supersonic to subsonic shock waves form and this decreases A/c performance.

Max Altitude up to which a Jet engine is flown depends upon the altitude at which IAS of 1.30% of Vmd will give you a Mach no. Just below Mach Critical.

Therefore Optimum altitude for Jet Engine A/c varies from day to day.

On a long cross country flight, as weight decreases IAS increases & we won’t be flying at 1.30% of Vmd, as the airspeed tends to increase at a lighter weight, therefore we climb up to a higher altitude, which further increases A/c performance by the increased TAS.

Therefore As weight decreases we climb to a higher altitude in a Jet Engine.

Therefore In a jet engine on long flights A/c climbs, using step up climb till it reaches the altitude at which Mach Critical affects further climb.

EPR- Engine Pressure Ratio means the pumping pressure ratio of the whole gas generator.

EPR is ratio of pressure at exit of the Compressor drive to the pressure at the Compressor inlet.

Example- 66÷100= 0.66%, 200÷100= 2

Pressure ratio of a gas generator is the ratio of pressure at the compressor exit to that at the compressor inlet.

Pressure ratio is at maximum rpm 6 to 1.

Thrust Reversers

  1. Clamshell type doors- operates pneumatically, reverse the exhaust gas-stream. The ducts are opened to deflect exhaust gases.
  2. Bucket Target System- operates through conventional pushrod system or is hydraulically powered, it uses bucket type doors to reverse the hot gas stream.
  3. Cold Stream Reverser System- operates by air motor or by hydraulic rams. The movable cowling moves rearward, the blocker doors fold and blank off the cold stream nozzle, diverting the airflow through the cascade vanes.

Cascade- are Baffles, which when the pilot selects the reverse thrust by putting the throttle control into reverse thrust detent, deflects the jet blast forward to achieve the reverse thrust.

THRUST AUGMENTOR

Water methanol is used to increase (water methanol is injected in compressor). It is used in Sub-sonic Aircraft compressor. When used the power produced is called Wet power. It increases the mass airflow by decreasing Temperature. Therefore value of exhaust Temperature gases can be exceeded.

In Super Sonic Aircraft reheat/afterburners are used fuel is injected in exhaust nozzle and burnt.

To operate thrust augmenter.

  1. Switch – ON
  2. Throttle should be at max forward position.

After Takeoff on decreasing throttle they automatically turn OFF.

At full power water Methanol is never switched OFF or Compressor will surge.

Airframe stresses

Tension due to pressurization.

In jet engine exhaust cone is followed by a exhaust nozzle.

Purpose of exhaust nozzle is to gather all the escaping gases and vent the out as a solid stream.

Exhaust cone is a divergent it decreases velocity and increases pressure.

Hot start- when air is less and fuel is more, temperature shoots up due t extra fuel and damages the engine.

Because airflow is less and fuel flow is more.

Hot start may lead to hung start.

It takes place in summer because Temperature is high and air density is less.

On starting H.P. compressor is started 1st because weight is less.

Gross Thrust – Gross thrust is momentum of outgoing gases .

Net Thrust – Net thrust is momentum of out going gases- momentum of incoming gases- momentum of fuel.

Some decrease in pressure takes place at all parts of jet engine that will cause a decrease in Thrust, but this is over come by the momentum of fuel.

Therefore Net Thrust = Mom of outgoing gases – Momentum of incoming gases only.

Gross thrust may be equal to Net thrust when Aircraft is stationery on the ground because momentum of incoming gases is 0.

Net Thrust= Momentum of outgoing gases – Momentum of incoming gases.

Gross Thrust= Mom of outgoing gases.

As airspeed increases Net thrust continues to decrease, but Gross thrust continues to increase, due to Ram effect. Therefore as Airspeed increases Total Thrust increases.

Specific Fuel Consumption for jet engine is lbs of fuel per hour ÷ lbs of Thrust.

S.F.C. for piston engine – fuel per hour ÷ S.H.P.

In static conditions 2.6 lbs of Thrust is taken as equivalent to 1Horse Power.

Thrust specific fuel consumption is Engine fuel flow ÷ Net Thrust

It is the amount of fuel required to produce one pound of Thrust.

Calorific value of Petrol is 76,000 Centigrade Heat Unit/gal.

Calorific value of Kerosene is 83,000 Centigrade Heat Unit/gal.

C.H.U. is heat necessary to raise one pound of water at the temperature of maximum density through one degree centigrade.

Kerosene has a low vapour pressure & will boil only at extremely high altitude or high temperatures, whereas wide cut fuel will boil at much lower altitude.

In a piston engine, power remains same, but as velocity increases T.H.P. increases up to a limit & then decreases.

In a jet engine, Thrust remains constant, but as velocity increases T.H.P. decreases.

At speed of 250kts/hr a jet give 1 lbs of Thrust = 1 H.P.

On doubling speed 500kts/hr a jet give 2lbs of Thrust = 2 H.P.

In a Subsonic flow Divergent passage is used, because it causes Velocity to decrease and pressure to increase

In a Super sonic flow if a Divergent passage is used, it causes velocity to increase pressure to decrease.

Therefore for Supersonic flow a Convergent passage is used, since it causes Velocity to decrease and pressure to increase.

But all super sonic A/c first start flying at Subsonic speed before operating at Super sonic speeds, therefore, Super sonic A/c has variable size air Intake.

A compressor converts Mechanical energy into pressure energy.

When we increase the pressure of air there is an Adiabatic rise in temperature. That means some energy is lost due to increasing temperature, therefore in a jet engine, the practical rise of pressure is less than Theoretical rise.

Rotor – Rotates & increases velocity & decreases pressure (Practically increase pressure also).

Stator – Stationary & decreases velocity & increases pressure & directs air to the next stage at an optimum angle of attack.

When you open Throttle abruptly first Low Pressure than High Pressure compressor stalls.

When throttle is closed abruptly all the stages stall instantly.

The compressor efficiency depends upon temperature, if temperature decreases compressor efficiency increases.

Therefore As altitude increases compressor efficiency increases.

First part of the turbine is Nozzle Guide Vanes.

Nozzle guide vane is the hottest part of a jet engine.

Two types of Turbines

1)    Impulse Reaction – Turn due to aerodynamic reaction.

2)   Impulse Turbine – runs at a lower temperature than a Reaction Turbine.

JPT – Jet Pipe Temperature.

TGT – Turbine Gas Temperature.

EGT – Exhaust Gas Temperature.

ITT – Inter Stage/Intermediate Turbine temperature.

TIT – Turbine Inlet temperature measured of N.G.V.

During starting cycle T.I.T is the highest temperature.

The temperature of exhaust is increased by-

1)   by induction of heater Methanol (wet power) in Compressor.

2)   By after burner/heater – Inducing fuel in exhaust section.

Gross Thrust = Momentum of outgoing gases.

Net Thrust = Momentum of outgoing gases – Momentum of Incoming gases – Momentum of fuel.

Therefore Net Thrust = Momentum of outgoing gases – Momentum of Incoming gases.

Static Thrust – Momentum Thrust produced by engine when A/c is stationary on the ground.

Jet engine efficiency depends upon Thrust Specific Fuel Consumption.

It is Thrust per hr per lbs of Fuel Consumption.

Factors affecting Jet engine efficiency.

1)   Airspeed increases gross thrust is same, Net thrust is less.

As airspeed increases, Net Thrust decreases up to 250 kts, but then due to Ram affect Net thrust increases.

Ram effect – Air at high velocity, hit the mass & increases pressure.

2)   Mass Airflow – increases thrust increases.

Mass Airflow depends upon maximum rpm.

RPM increases mass airflow increases therefore jet engines are operated at high rpm.

Tachometer of a jet engine is calibrated in the percentage of maximum rpm.

Factors Affecting Mass airflow.

(a)   Temperature increases thrust output decreases, Altitude increases, thrust output decreases, but jet engine efficiency increases, because compressors are more efficient at cold temperature (It takes less energy to compress cold air as compared to compressor hot air).

(b)  Press increases, Mass Airflow increases.

(c)   Altitude increases, Mass Airflow decreases.

Hottest Part of Jet engine serially is

1)   Nozzle Guide Vanes.

2)   Turbines

3)   Exhaust Valve

4)   Compressors.